SNCR has traditionally been applied to industrial and utility boilers, incinerators, and process heaters for the reduction of nitrogen oxide emissions from lean burn combustion sources. In SNCR, a reagent such as aqueous ammonia or aqueous urea is injected into a furnace zone where the temperature is typically 1700-2200 F. The reagent decomposes in the hot furnace gas and goes through a number of chemical reactions that convert nitrogen oxides into water vapor and nitrogen gas without the use of a catalyst.
Hundreds of SNCR systems are in commercial service around the world and the basic SNCR chemistry is well known to those skilled in the art. While it is lower in capital cost than selective catalytic reduction (SCR) of NOx systems, SNCR suffers from lower levels of NOx reduction and poor chemical utilization in part due to temperature variation and unequal NOx spatial distribution across the large dimensions of a furnace. Incomplete chemical reactions lead to secondary pollutants such as ammonia slip (NH3) and carbon monoxide (CO).
It has been traditional to inject a very dilute solution of the reagent in water, with a reagent concentration in the range of 2-10%, and may be as low as 2% to accommodate the high temperatures and the large dimensions across a furnace. Water provides cooling of individual water/reagent droplets as well as mass and momentum to the droplets so that they penetrate across the furnace. In a typical SNCR system, a chemical circulation pump circulates an aqueous solution of 32-50% urea from a bulk storage tank over to a chemical metering and mixing skid where the aqueous based reagent is further diluted with additional water and pumped to separate injector distribution modules (IDM). At each distribution module, the diluted reagent is further split to supply a number of individual injectors. The chemical flow rate to each injector may be monitored and adjusted at the injector distribution module, but the reagent concentration, which comes from the common mixing skid, is fixed and the same to all injectors. The diluted reagent may by atomized by air along the supply line and/or at the injector. The atomizing airflow to each injector may also be monitored and adjusted at the injector distribution module using standard valves.
The dilution rate of an aqueous reagent in the chemical mixing skid may be controlled by using a control valve or a variable speed pump. When the dilution rate changes, the concentration of the reagent solution in the mixing skid changes, yet the concentration of the reagent solution flowing from the mixing skid at any time is the same to all distribution modules and to all injectors. But in reality, the NOx concentration in any one section of the furnace may vary from section to section and from elevation to elevation, especially as load or fuel or the fouling of heat transfer surfaces changes in the furnace. The temperature profile in the furnace also shifts with load. Thus, there is a need for dynamically adjusting the amount of NOx reducing agent fed into different positions of a furnace.
The art has continued to seek methods of improving the NOx reduction performance and chemical (e.g., ammonia, urea) utilization in a SNCR process while preventing the production of other pollutants.
U.S. Pat. No. 4,780,289 to Epperly discloses a process for NOx reduction in an effluent from the combustion of a carbonaceous fuel while minimizing the production of other pollutants. The process comprises determining a NOx reduction versus effluent temperature curve for each of a plurality of treatment regimens, and introducing (most commonly by injecting) a NOx treatment agent into the effluent according to a NOx reducing treatment regimen such that the treatment agent is operating on the high temperature or right side of its NOx reduction versus effluent temperature curve for an efficient NOx reduction. Epperly teaches that adjusting dilution/introduction rate and relative presence of enhancers of the treatment agent will shift the curve and thereby cause the introduction of the treatment agent to operate on the right side of the curve.
U.S. Pat. No. 4,777,024 to Epperly is directed to a multi-stage process for reducing the concentration of pollutants in an effluent. Treatment agents are injected into the effluent of different temperature zones, respectively, to reduce the concentration of nitrogen oxides in the effluent from the combustion of a carbonaceous fuel. The treatment agents include urea/ammonia and an enhancer selected from a group of specific compounds. But the cost, availability, and storage considerations of the enhancer make the already complicated multi-stage process very unattractive.
Furthermore, U.S. Pat. No. 4,830,839 to Epperly describes a process for ammonia scrubbing by use of a non-nitrogenous treatment agent.
U.S. Pat. No. 5,252,298 to Jones takes a different approach for improving NOx reduction efficiency. Jones describes an apparatus for injecting reagents into a combustion effluent through a nozzle, wherein the nozzle may be aimed in response to the temperature of an effluent. In a preferred embodiment, four injector assemblies are used with equal quantities of injection mixture from each nozzle.
U.S. patent application Ser. No. 10/290,797 to Valentine teaches the use of a metering valve to introduce a total volume of dilution water and reagent to a SNCR lance through an injector tip in a SNCR process. Unfortunately, Valentine fails to recognize the high level of dilution water required in a SNCR reagent injection and the physical limitations of readily available metering valves of the automotive fuel injector type (i.e., a solenoid actuated metering valve) proposed for use by Valentine. In SNCR applications, the reagent concentration in water is typically less than 10% and often is only 2-5%. The injection rate of combined dilution water and reagent for each SNCR lance is typically in the range of 1.0-1.5 gpm (gallons per minute), or 60-90 gallons per hour of mixed liquid per SNCR lance. However, a solenoid actuated metering valve has a high-end injection rate of 7-10 gph and perhaps up to 15 gph. Thus, the Valentine method falls short of being practical using commercially available small capacity valves. The Valentine method would require the use of, and/or the development of, a much higher capacity type of metering valve than a solenoid actuated metering valve.
Therefore, there is still a need to provide a system and method for improving SNCR performance. Desirably, the system and method are able to adjust the injection rate of a NOx reducing agent at each SNCR injector to better match the reagent injection to the local NOx concentration across the furnace, while at the same time maintaining relatively constant water and air flow to the injectors to keep the droplet size and penetration into the furnace consistent. It would also be desirable for the injectors placed on the furnace wall to be pivotable with respect to the wall surface so as to target the reagent injection to a preferred temperature zone inside the furnace.